1. Field of the Invention
The invention relates to a current measurement circuit, and more particularly, to a current measurement circuit which measures current over a wide dynamic range.
2. Description of the Related Art
Current measuring circuits are required in many devices. FIG. 1 shows a conventional current measurement circuit 100 disclosed in U.S. Publication 2008/0303507, which may be applied in thin film applications such as active matrix displays. The current measurement circuit 100 comprises a charge integrating circuit 2, a comparator circuit 4 and a logic circuit 6. The charge integrating circuit 2 integrates a charge from a current 10 to be measured and applies a change in voltage to a comparator circuit 4. The comparator circuit 4 compares the input voltage 12 with a threshold voltage level and provides an output 14 responsive thereto to the logic circuit 6. The logic circuit 6 generates a feedback signal 16 according to the comparator output 14 and provides the feedback signal 16 to the charge integrating circuit 2. Then, the charge integrating circuit 2 integrates a charge from the received feedback signal 16, which is opposite to the integrated charge from the current 10.
In FIG. 1, an integration time for integrating the current 10 is proportional to the current 10 being measured. For example, when a small current is being measured, the integration time is long and vice versa. A relatively long setup period is required, which is used to accurately set the input voltage 12 of the comparator 4 at a threshold value.
Furthermore, in FIG. 1, a feedback capacitor C is coupled to and between the input of the comparator circuit 4 and the output of the logic circuit 6. During a measurement period, the voltage applied to the feedback capacitor C by the logic circuit 6 is stepped, which will produce a step-up voltage at the input of the comparator circuit 4. Ideally, the voltage at the input of the comparator circuit 4 should be constant so that the current 10 to be measured is independent of the resistance of the current source 60 and transient currents which can occur in thin film devices due to sudden changes in a bias voltage may be prevented.
Therefore, a current measurement circuit for decreasing the duration of the initial integration period (setup period), reducing the difference in measurement time for large and small currents and reducing the step-up voltage at the input of the comparator circuit is desired.